Oh that is a tough question!
I'll always have a soft spot for Hellfire, but that IS mostly from a nostalgic 'I was so excited at the time' kinda thing. Other zones are prettier (both in style and graphics) and play better... But because it has been so unchanged, you HAVE to do that starting zone, and because I've done it so many times I know it so well, both horde and alliance versions.
Howling Fjord has to be up there, looks so glorious, the quests, for the most part, flowed nicely and were diverse enough to keep entertained, definitely prefered it over Borean Tundra! (kinda upset that on my current monk play through I ended up in borean tundra without thinking about it, would much rather have done howling fjord..)
With the Cata starting zones it was less clear cut, I was pretty excited for leveling my druid main through Hyjal, And it did look nice and all that, the quests flowed near perfectly around the zone without needless travelling etc. But I think its downside is that its too long, by the end of it you're just like "I'm done, can we not just move on already" But I'm not sure what it is thats missing, it felt like just trudging through quests, rather than the immersive wonderfest it should've been... But I did prefer it over Vash'jir (though i'm not an underwater zone hater, but there was bits in Vash'jir that bugged me)
And last, but not least, Jade Forest, Another zone I really like! Scenery is beautiful, quests are a bit more immersive and they're interestingly varied and fun. I prefered the Jinyu quest hub to the Hozen one though.
So from worst to favourite:
Borean Tundra
Vash'jir
Hyjal
Jade Forest
and tied for first
Hellfire Peninsula and Howling Fjord.